This invention relates to devices for pulling crystals such as mono-crystals and in particular straight mono-crystals from a molten compound such as a semiconductor compound containing a volatile element such as phosphorus, arsenic, selenium or sulphur in general and more particularly to an improved monitoring device for use in such a device.
In a recently developed crystal pulling device useful for pulling mono-crystals, and particular straight mono-crystals from a molten compound, preferably a semiconductor compound which contains a volatile component such as phosphorus, arsenic, selenium or sulphur, the device includes a high pressure drawing chamber arranged on a stand with the drawing spindle of a crystal mount led through the cover of the stand. The stand, the high pressure drawing chamber and the crystal mount are disposed one above the other symmetrical to the drawing axis. The crystal mount and the cover of the high pressure drawing chamber, which together with the side walls of the chamber are in the form of a bulb, form a structural unit mounted on the stand so as to be moveable in the direction of the drawing axis. The device is particularly well suited for the pulling crystals using the protective bath method. However, it is also applicable to other drawing methods such as the well known multiple temperature method.
In this crystal pulling device a monitoring arrangement with several observation windows is provided in the high pressure drawing chamber. A television camera common to all the observation windows is coordinated with them. Difficulties have arisen with this arrangement. It has turned out that the great temperature between the crucible, heated to approximately 1500.degree. C and containing the melt in a protective bath, and the high pressure drawing chamber bulb, which is generally only a few e.g., about 2 cm, away and is cooled practically to room temperature, leads to a substantial termodiffusion of an inert gas contained in the chamber. This thermodiffusion hinders the observation of the crystal growth and thereby also hinders control of the pulling device during the drawing operation. It should be noted that the control of the pulling operation requires that even slight changes in the solid-liquid phase boundary and slight changes on the surface of the pulled crystal be accurately and exactly observed.
Although the pulling operation is carried out under considerable overpressure, preferably about 60 atm or more, which overpressure to a large degree prevents evaporation of the volatile component of the semiconductor compound, a condensation of this component on the observation windows can occur. As a result a cumbersome disassembly, cleaning and reassembly of the observation windows after each pulling operation can be necessary. Such is relatively costly. Furthermore it is quite possible that the relatively complicated, pressure tight observation windows can be damaged during repeated disassembly and reassembly.
In view of these difficulties, it is the object of the present invention to provide an improved monitoring device for monitoring such a process.